Fuel injection systems deliver fuel to the combustion chamber of an engine, where the fuel is mixed with air before combustion. One form of fuel injection system known in the art is a fuel spray nozzle. Fuel spray nozzles atomise the fuel to ensure its rapid evaporation and burning when mixed with air.
An airblast atomiser nozzle is a type of fuel spray nozzle in which fuel delivered to the combustion chamber by a fuel injector is aerated by swirlers to ensure rapid mixing of fuel and air, and to create a finely atomised fuel spray.
Efficient mixing of air and fuel results in higher combustion rates. It also reduces unburnt hydrocarbons and exhaust smoke (which result from incompletely combusted fuel) emitted from the combustion chamber.
Additionally, “lean burn combustion” is being developed as a way of operating at relatively low flame temperatures. The lower temperatures significantly reduce NOx emissions, but can necessitate the use of a pilot and mains fuel nozzle to avoid lean extinction at low engine powers.
FIG. 1 shows a schematic view of a fuel injection nozzle 10 which, in use, would be mounted on the upstream wall of a combustion chamber 100.
The fuel injection nozzle 10 has a central axis 11, and is in general circularly symmetrical about this axis. A pilot fuel injector 12 is centred on the axis, and is surrounded by a pilot swirler 13. A mains airblast fuel injector 14 is concentrically located about the pilot fuel injector 12, with inner and outer mains swirlers 15 and 16 positioned radially inward and outward thereof.
The mains airblast fuel injector has an annular flow passage or gallery 17. Circumferentially spaced fuel distributor slots 19 deliver fuel to the fore end of the gallery. The fuel is then conveyed along the gallery to a prefilming lip 18 formed at the aft end of the gallery. An annular film of liquid fuel forms on the lip, and is entrained in and atomised by the much more rapidly moving and swirling air streams produced by inner mains swirler 15 and outer mains swirler 16.
To achieve lean burn, the system not only incorporates pilot and mains fuel injectors, but also requires a relatively large amount of combustion air. To realise the low combustion temperatures the fuel must be well mixed with the air prior to combustion, hence creating uniform low flame temperatures. Non-uniform mixing prior to combustion can result in locally high combustion temperatures, and hence no reduction in NOx emissions. Low combustion efficiency in the lower temperature areas increases the engine's specific fuel consumption, and emissions of carbon monoxide and unburnt fuel.
Thus, it is desirable to improve the design of fuel injectors to achieve more uniform fuel-air mixing.